It is well known as a conventional technique, in an intake system of an internal combustion engine, to reduce intake air pulsation caused by reciprocating movement of a piston and an intake valve by regulating the diameter and length of an intake passage such as an intake duct or the like, the volume of an air cleaner constituting a part of the intake system and by adding a resonator to the intake system.
However, in such a known technique, tuning air intake sound within a preset certain target sound pressure characteristics range is the main object. Accordingly, the sound pressure characteristics of this tuned air intake sound does not necessarily change linearly with an increase in engine speed, so that there is a fear of providing uncomfortable feeling to a driver.
A patent literature 1 discloses obtaining air intake sound having powerful feeling within a passenger compartment during acceleration by connecting a plurality of intake air ducts having resonance frequencies different from each other to a dusty side of an air cleaner and by generating air intake sound in these intake air ducts during an engine high load operation.
The patent literature 1: Japanese Patent Provisional Publication No. 2000-303925.
In this patent literature 1, a characteristics in which the frequency and sound pressure level increases generally linearly with an increase in engine speed can be obtained within the passenger compartment during acceleration by using air intake sounds generated from the intake air ducts during the engine high load operation. However, in the arrangement of the patent literature 1, it is necessary to tune the length of the intake air duct, the passage cross-sectional areas inside the intake air ducts and the like in order to generate desired air intake sound during the engine high load operation, and therefore there is a fear that a layout of the intake system is largely restricted.
Additionally, the following technique has been hitherto known: In the intake air passage through which necessary air is taken into the internal combustion engine, a part of air intake sound leaks to an outside section (or the outside of the intake air passage) through a part of a peripheral wall of the intake air passage, thereby lowering the sound pressure at a specified frequency so as to achieve silence. For example, a patent literature 2 discloses an intake air duct which is arranged such that an opening portion is formed at a part of the peripheral wall of the intake air duct through which outside air is introduced into an air cleaner, and a porous material having an air permeability is joined to cover this opening portion.
The patent literature 2: Japanese Patent Provisional Publication No. 2001-12324.
However, in this patent literature 2, there is such a problem that the sound pressure level become rather too low at a part of frequency ranges because of being arranged from such a viewpoint that the sound pressure level is sufficient to meet a certain target level only on the basis of taking preference of silence of the intake system, so that the tone feeling of air intake sound felt by the driver within the passenger compartment becomes unsatisfactory to the driver.
In view of the above, an object of the present invention is to provide an intake device of an internal combustion engine, for preventing the tone feeling of air intake sound from degrading.